Described herein are a method, a system, and an apparatus for sintering metal components or metal alloy components, particularly steel components. More particularly, described herein are a method, a system, and an apparatus for sintering steel components.
Powder metallurgy is routinely used to produce a variety of simple- and complex-geometry carbon steel components requiring close dimensional tolerances, good strength and wear resistant properties. This process, also known as sinter hardening, typically is used to produce iron-based alloys which exhibit high hardness through consolidating and sintering metallurgical powders. The process involves pressing metal powders that have been premixed with organic lubricants into useful shapes and then sintering them at high temperatures in continuous furnaces into finished products in the presence of controlled atmospheres. The controlled atmosphere for this process typically contains nitrogen and hydrogen or an endo gas mixture.
The continuous sintering furnaces normally contain three distinct zones, i.e., a preheat zone, a hot zone, and a cooling zone. The preheat zone is used to preheat components to a predetermined temperature and to thermally assist in removing organic lubricants from components. The hot zone is used to sinter components. The temperature of the hot zone typically ranges from 600° C. to 1350° C. However, this temperature may vary depending upon the metal powders being processed. The cooling zone is used to cool components prior to discharging them from continuous furnaces. In the cooling zone, transformation to the martensite phase may occur.
Sintering of metals including sinter hardening of steels under inert and reducing atmospheres are well known and established. A comprehensive review of technological factors controlling sinter-hardening may be found in “Effect of Cooling Rates During Sinter-Hardening” by G. Fillari et al., presented at PM2TEC 2003, Las Vegas, Nev., “A review of current sinter-hardening technology” by M. L. Marucci et al., presented at PM2004 World Congress, Vienna, Austria, “Sintering a path to cost-effective hardened parts” published in Technical Trends, MPR June 2005, 0026-0657/05© 2005 Elsevier Ltd., and in the 2009 publication titled: “Influence of Chemical Composition and Austenitizing Temperature on Hardenability of PM Steels” by P. K. Sokolowski and B. A. Lindsley, PowderMet 2009, 2009 Int. Conf. on Powder Metallurgy & Particulate Materials, June 28-July 1, Las Vegas, Nev.
The cooling temperature and rate is important in controlling the final properties of the end product such as surface hardness, hardness, tensile strength, and/or sintered density. One method of improving one or more of these properties is to add one or more alloying materials to the metal powder composition to control its phase transformation. For example, for certain sinter hardenable materials, delaying the austenite to ferrite plus carbide transition to form martensite may increase the hardenability. As hardenability increases, martensite may form at progressively lower cooler rates. Examples of suitable alloying materials include, but are not limited to, manganese (Mn), chromium (Cr), molybdenum (Mo), copper (Cu), nickel (Ni), and combinations thereof. Higher levels of alloying additions increases the costs associated with raw materials of the parts. Moreover, higher levels of alloying additions in powder metallurgy parts may reduce powder compressibility which, in turn, affects the capital and operating costs of operations.
Other methods of overcoming the problem of low cooling rates in the continuous, sintering and sinter hardening furnaces, in addition to, or as an alternative of elevated levels of alloying additions in the parts processed, include using pure hydrogen or H2-rich furnace atmospheres to accelerate heat transfer. However, the use of the H2 atmospheres increases operating as well as capital costs due to the H2 cost and safety risks involved in handling explosive gases. Low cooling capacity of the conventional, convective cooling systems used in the industrial practice today creates, additionally, a bottleneck in the production process because fewer parts can be run through continuous furnace at once, or lower processing speeds need to be used, in order to cope with the task of affecting heat removal in the cooling zone.
Thus, one of the key challenges in sinter-hardening and other heat treating operations is to provide sufficient part cooling rates in the cooling zone to produce a martensitic phase transformation and obtain the desired hardening effect. The conventional, convective gas-cooling systems installed in the continuous sintering furnaces are significantly less efficient than the conventional oil, polymer, salt, or water quenching baths and high-pressure gas quenching systems that are preferred in batch-type heat treating operations. The use of quenching baths in the continuous furnace operations would, nevertheless, be impractical, and the use of high-pressure gas quenching cells extremely limited.
There is a need in the art to improve the cooling profile in a sinter hardening process without necessitating the addition of one or more expensive alloying materials, or alternatively, reducing the amount of alloying materials added.